Trench IGBT with waved floating P-well electron injection

ABSTRACT

A trench IGBT includes a floating P well and a floating N+ well that extends down into the floating P well. A bottom surface of the floating P well has a novel waved contour so that it has thinner portions and thicker portions. When the IGBT is on, electrons flow from an N+ emitter, vertically through a channel along a trench sidewall, and to an N− type drift layer. Additional electrons flow through the channel but then pass under the trench, through the floating P well to the floating N+ well, and laterally through the floating N+ well. NPN transistors are located at thinner portions of the floating P type well. The NPN transistors inject electrons from the floating N+ type well down into the N− drift layer. The extra electron injection reduces V CE(SAT) . The waved contour can be made without adding any masking step to an IGBT manufacturing process.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of, and claims priority under35 U.S.C. 120 from, nonprovisional U.S. patent application Ser. No.14/840,322, entitled “IGBT With Waved Floating P-Well ElectronInjection”, filed on Aug. 31, 2015, by Kyoung Wook Seok. Thisapplication incorporates by reference the entire contents of U.S. patentapplication Ser. No. 14/840,322.

TECHNICAL FIELD

The described embodiments relate to trench Insulated Gate BipolarTransistors (IGBTs).

BACKGROUND INFORMATION

In an Insulated Gate Bipolar Transistor (IGBT), it is generally the casethat increasing the concentration of charge carriers, both electrons andholes, in the N− type drift layer of the IGBT, and maintaining theproper balance and distribution of holes to electrons in the N− typedrift layer, serves to reduce the collector-to-emitter saturationvoltage V_(CE(SAT)) of the IGBT. IGBT structures are desired that havehigh concentrations of electrons and holes in their drift regions duringthe IGBT on state, but yet turn off fast and do not suffer latchup andother problems. U.S. patent application Ser. No. 14/840,322, entitled“IGBT With Waved Floating P-Well Electron Injection”, filed Aug. 31,2015, by Kyoung Wook Seok sets forth several planar IGBT structures.

SUMMARY

A trench IGBT structure includes a floating P type well region down intoan N− type drift layer, and a floating N+ type well region that extendsdown from an upper semiconductor surface into the floating P type wellregion. A bottom surface (boundary with the N− type drift layer) of thefloating P type well region has a novel waved contour so that thefloating P type well region has thinner portions and thicker portions.The thinner portions extend to a depth DP2THIN, where DP2THIN ismeasured from the upper semiconductor surface. The thicker portionsextend to a depth DP2THICK, where DP2THICK is measured from the uppersemiconductor surface. In one example, the thinner portions of thefloating P type well region are less than half as thick as the thickerportions of the floating P type well region. Where the depth of thefloating N+ type well region is DN, the quantity DP2THIN minus DN isless than half the quantity DP2THICK minus DN.

When the trench IGBT is on, electrons flow from an N+ type emitterregion, vertically through a conductive channel along a trench sidewall,and to the N− type drift layer. In one novel aspect, some electrons flowthrough the channel but then pass laterally under the trench, into thefloating P type well region, up to the floating N+ type well region, andthen laterally through the floating N+ type well region. Localelectron-injecting NPN transistors are located at the thinner portionsof the floating P type well region. Base current for these local NPNtransistors is supplied in the form of hole flow, where the holes passupward from the N− drift region into the floating P type well region,and then pass into the thinner base portions of the floating P type wellregion (thereby constituting base currents for the NPN transistors), andthen pass up into the floating N+ type well region (at the emitters ofthe local transistors). These holes then pass laterally in the N+ typewell region for a distance toward the trench edge of the floating N+type well region, but they combine with some of the electrons of themuch larger electron flow in the opposite direction. In the IGBT onstate, these local NPN transistors turn on and inject electrons from thefloating N+ type well region down into the N− type drift layer. Theextra electron injection, which occurs in the areas of the thinnerportions, serves to reduce V_(CE(SAT)) of the trench IGBT in the IGBT'son state.

In some examples, the waved contour of the bottom boundary of thefloating P type well region is made without adding any masking step to atrench IGBT manufacturing process. The same ring mask used to define andto form floating P type rings in an edge termination area of the trenchIGBT is also used to define and to form the thinner portions of thefloating P type well region. Spacings between features of this ring maskcan be adjusted and set so as to set DP2THIN, and to set the shape andwidth of the thinner portions. In one example, a thinner portion of thefloating P type well region has a closed polygonal ring shape when thetrench IGBT die structure is considered from the top-down perspective.Multiple such thinner portions in one example form a set of concentricpolygonal rings when the trench IGBT die structure is considered fromthe top-down perspective. In one example, the floating P type wellregion forms a part of a sidewall of a trench, and the floating P typewell region at all locations along this trench extends from the uppersemiconductor surface to a depth greater than DP2THIN.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a top-down diagram of the semiconductor surface of a trenchIGBT die structure in accordance with one novel aspect.

FIG. 2 is a cross-sectional diagram taken along sectional line A-A′ ofFIG. 1.

FIG. 3 is a cross-sectional diagram taken along sectional line A-A′ ofFIG. 1, where the diagram sets forth dimensions, dimensionalrelationships, and illustrates electron injection from localelectron-injector NPN bipolar transistors.

FIG. 4A is a cross-sectional diagram that illustrates a first step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4B is a cross-sectional diagram that illustrates a second step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4C is a cross-sectional diagram that illustrates a third step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4D is a cross-sectional diagram that illustrates a fourth step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4E is a cross-sectional diagram that illustrates a fifth step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4F is a cross-sectional diagram that illustrates a sixth step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4G is a cross-sectional diagram that illustrates a seventh step ina method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4H is a cross-sectional diagram that illustrates an eighth step ina method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4I is a cross-sectional diagram that illustrates a ninth step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4J is a cross-sectional diagram that illustrates a tenth step in amethod of manufacturing the IGBT die structure of FIG. 1.

FIG. 4K is a cross-sectional diagram that illustrates an eleventh stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4L is a cross-sectional diagram that illustrates a twelfth step ina method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4M is a cross-sectional diagram that illustrates a thirteenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4N is a cross-sectional diagram that illustrates a fourteenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4O is a cross-sectional diagram that illustrates a fifteenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4P is a cross-sectional diagram that illustrates a sixteenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4Q is a cross-sectional diagram that illustrates a seventeenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4R is a cross-sectional diagram that illustrates an eighteenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4S is a cross-sectional diagram that illustrates a nineteenth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4T is a cross-sectional diagram that illustrates a twentieth stepin a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4U is a cross-sectional diagram that illustrates a twenty-firststep in a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4V is a cross-sectional diagram that illustrates a twenty-secondstep in a method of manufacturing the IGBT die structure of FIG. 1.

FIG. 4W is a cross-sectional diagram that illustrates a twenty-thirdstep in a method of manufacturing the IGBT die structure of FIG. 1.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings. In the description and claims below, when a firstobject is referred to as being disposed “over” or “on” a second object,it is to be understood that the first object can be directly on thesecond object, or an intervening object may be present between the firstand second objects. Similarly, terms such as “upper”, “top”, “up”,“down”, “vertically”, “laterally”, “lower”, “bottom”, and “backside” areused herein to describe relative orientations between different parts ofthe structure being described, and it is to be understood that theoverall structure being described can actually be oriented in any way inthree-dimensional space. The notations N+, N−, N, P++, P+, and P areonly relative, and are to be considered in context, and do not denoteany particular dopant concentration range. A region denoted generally inthe claims to be “P type”, however, is being indicated to be P typedoped, and may be lightly doped, moderately doped, or heavily doped withP type dopants. Similarly, a region denoted in the claims to be N typeis being indicated to be N type doped, and may be lightly doped,moderately doped, or heavily doped with N type dopants.

FIG. 1 is a top-down diagram of a square part of a central active areaof an IGBT die structure 2 in accordance with one novel aspect. Thetop-down diagram is of the upper semiconductor surface 8 of the IGBT diestructure 2. Overlying layers, such as passivation, metallizationlayers, and oxide layers, are omitted from FIG. 1 so that the underlyingsemiconductor structures will not be obscured in the diagram. Thecentral active area has a repeating structure and pattern as illustratedin FIG. 1. Surrounding this repeating structure and pattern in theactive area is a peripheral edge termination area (not shown). Theperipheral edge termination area includes, among other edge terminationstructures, multiple concentric floating P type guard rings (not shown).These floating P type guard rings ring around the central active area,and extend along the outer square peripheral edge of the die.

FIG. 2 is a cross-sectional diagram taken along sectional line A′-A inthe top-down diagram of FIG. 1. The vertical line labeled A at the leftof FIG. 1 corresponds to a central location A in the structure of FIG.2.

The trench IGBT die structure 2 includes an N+ type buffer layer (alsocalled a “field stop” layer) 3 that is disposed over the top majorsurface 4 of a P++ type semiconductor substrate layer 5. An N− driftlayer 6 is in turn disposed over the N+ type buffer layer 3. A P typebody region 7 is formed to extend down into the N− type drift layer 6.The P type body region 7 has a relatively lighter doped P type portionand a relatively heavily doped P+ type portion 10. An N+ type emitterregion 11 is formed to extend from a substantially planar uppersemiconductor surface 8 down into the P type body region 7.

In addition, a floating P type well layer or region 12 is formed toextend down into the N− drift layer 6. The floating P type well region12 is laterally separated from the P type body region 7 by a trench 17.As seen in the top-down diagram of FIG. 1, this trench actually extendsall the way around the floating P type well region 12. A floating N+type well region or layer 13 is formed to extend down into the floatingP type well region 12 from the upper semiconductor surface 8 asillustrated. The trench 17 extends downward from the upper semiconductorsurface 8 as illustrated. The square area (as seen from the top-downdiagram of FIG. 1) of the two floating regions 12 and 13 may also bereferred to as a “dummy” cell in that it looks somewhat like anothercell of the multi-cell structure, but it does not have an N+ typeemitter region like region 11. Even though the floating N+ type wellregion 13 in this particular example is disposed between the top of thefloating P type well region 12 and the upper semiconductor surface 8,the floating P type well region 12 is said to extend into the N− typedrift layer 6 “from” the surface 8 to indicate a direction oforigination consistent with the structure illustrated in FIG. 2.

A thin gate oxide layer 16 is formed on the sidewall surfaces and on thebottom surface of the trench. A gate electrode 14 of N+ type polysiliconfills the remainder of the trench as illustrated, and as is conventionalin trench IGBT manufacture. The N+ type polysilicon of the gateelectrode may have an N type dopant concentration in a range of from1×10¹⁹ atoms/cm³ to 1×10²¹ atoms/cm³. The trench IGBT die structure 2 ofFIG. 1 further includes an oxide layer 21. A first metal electrode andterminal 22 (the emitter terminal) is disposed over the oxide 21 and iscoupled both to the N+ type emitter region 11 and to the P type bodyregion 7 via the P+ ohmic contact region 10. The P+ type ohmic contactregion may have a P type dopant concentration of about 1×10¹⁹ atoms/cm³.The area shown in cross-hatching in FIG. 1 and identified by referencenumeral 31 is this metal-to-semiconductor contact region. A second metalelectrode and terminal 15 (the gate terminal) has a contact portion atthe top of the IGBT die structure and is coupled to the polysilicon gateelectrode 14. The second metal electrode and terminal 15 is not shown inthe cross-section of FIG. 2 because the contacts between the secondmetal electrode 15 and the polysilicon gate electrode 14 is outside theactive area. The gate terminal 15 is therefore represented in FIG. 2 bya terminal symbol. A third metal electrode and terminal 23 (thecollector terminal) is formed on the bottom major surface 24 of the P++type substrate layer 5. A passivation layer 18 covers the top of theIGBT die structure, but for exposed contact areas down to the emitterterminal metal and down to the gate terminal metal.

The floating P type well region 12 has a waved bottom interface 25 withthe underlying N− type drift layer 6. Due to the waved form of thisinterface 25, the floating P type well region has thinner portions 26and 27 as well as thicker portions 28, 29 and 30. The bottom of each thethinner portions 26 and 27 of the floating P type well region 12 is at adepth DPTHIN (measured from the upper semiconductor surface 8). Thebottom of each of the thicker portions 28, 29 and 30 of the floating Ptype well region 12 has a depth DPTHICK (measured from the uppersemiconductor surface 8). The P type body region 7 has a depth DP1(measured from the upper semiconductor surface 8). The trench 17 extendsdownward from the upper semiconductor surface 8 to a depth DT. Thefloating N+ type well region 13 and the N+ type emitter region 11 extenddownward from the upper semiconductor surface 8 to a depth DN. Thedimensions DN, DP2THIN, DP2THICK, and DP1 are illustrated on FIG. 3,along with dimensional relationships. In one example, each of thethinner portions of the floating P type well region is less than half asthick as the thicker portions of the floating P type well region.Accordingly, the quantity DP2THIN minus DN is less than half of thequantity DP2THICK minus DN. In one specific example, DN is 0.3 microns,DP2THIN is 0.5 microns, DP2THICK is 1.7 microns, DP1 is 1.7 microns, andDT is 2.0 microns. The locations of the thinner portions 26 and 27 areindicated in the top-down diagram of FIG. 1 by the two concentricsquares labeled 26 and 27. The first thinner portion 26 is a smallersquare ring as illustrated in FIG. 1. The second thinner portion 27 is alarger square ring that rings around and surrounds the smaller ring.

At the location of a thinner portion, a local electron-injecting NPNbipolar transistor structure is formed. For example, in the case ofthinner portion 26, a local portion of the floating N+ type well 13 isthe emitter, the thinner portion 26 of the floating P type well region12 is the base, and an amount of N− type semiconductor material of theN− type drift region 6 immediately below thinner portion 26 is thecollector. When these local electron-injecting NPN bipolar transistorsturn on, they can inject electrons downward from the N+ type emitterregion 13 into the N− type drift region 6 as described in more detailbelow.

The trench IGBT device is turned on by placing an appropriate positivevoltage on the gate electrode 14, thereby establishing a conductivechannel along the vertical sidewall trench between the N+ type emitterregion 11 and the N− type drift layer 6. In conventional IGBT fashion,electrons flow from the N+ type emitter region 11, vertically downwardthrough this channel through the P type body region 7, and to the N−type drift layer 6, and from there the electrons continue to passvertically downward deeper into the N− type drift layer 6. This electronflow is illustrated by heavy arrows 32 and 33 in FIG. 3. In addition, inconventional IGBT fashion, the PN junction between the P+ type collectorlayer 5 and the N+ type buffer layer 3 injects holes upward, and theholes pass up into the N− type drift layer 6. A high concentration ofelectrons and holes in the N− drift layer 6 forms. This highconcentration of electrons and holes is referred to as a plasma or as anelectron/hole gas. Overall, a collector-to-emitter current in the onstate of the IGBT flows from the collector electrode 23, vertically upthrough the device, and to the emitter electrode 22.

In addition to this conventional current flow in the IGBT on state,electrons also flow as indicated by arrows 34 and 35 in FIG. 3.Electrons flow from the N+ type emitter region 11, vertically downwardthrough the conductive channel, then laterally and horizontally underthe bottom of the trench 17, and then through the floating P type wellregion 12, and to the floating N+ type well region 13. These electronsflow horizontally through the N+ type well region, and then are injecteddownward by local NPN bipolar transistors. There is a first such localNPN bipolar transistor formed in the area of thinner portion 26. Theemitter of this first local NPN bipolar transistor is a part of thefloating N+ type well region immediately above the thinner portion 26.The base of this first local NPN bipolar transistor is the thinnerportion 26 of the floating P type well region. The collector of thisfirst local NPN bipolar transistor is the N− type material of the N−type drift layer 6 immediately beneath the thinner portion 26. Basecurrent for this first local NPN transistor is supplied in the form ofhole flow, where the holes pass upward from the N− drift region 6 intothicker portions of the floating P type well region, and then pass intothe thinner base portion 26 of the floating P type well region 12(thereby constituting a base current flowing into the base portion ofthe first local NPN transistor), and then pass up into the floating N+type well region (that is the emitter of the first local NPNtransistor). These holes then pass laterally in the floating N+ typewell region for a distance toward the trench edge of the N+ type wellregion, but they combine with some of the electrons of the much largerelectron flow in the opposite direction. In the IGBT on state, thisfirst local NPN bipolar transistor turns on and injects electrons fromthe floating N+ type well region 13 vertically downward as indicated byarrow 34.

In addition, there is a second such local NPN bipolar transistor formedin the area of thinner portion 27. The emitter of the second local NPNbipolar transistor is a part of the floating N+ type well region 13immediately above the thinner portion 27. The base of the second localNPN bipolar transistor is the thinner portion 27 of the floating P typewell region. The collector of the second local NPN bipolar transistor isthe N− type material of the N− type drift layer 6 immediately beneaththe thinner portion 27. Base current for this second local NPNtransistor is supplied in the form of hole flow, where the holes passupward from the N− drift region 6 into thicker portions of the floatingP type well region, and then pass into the thinner base portion 27 ofthe floating P type well region 12 (thereby constituting a base currentflowing into the base portion of the second local NPN transistor), andthen pass up into the floating N+ type well region (that is the emitterof the second local NPN transistor). These holes then pass laterally inthe floating N+ type well region for a distance toward the trench edgeof the N+ type well region, but they combine with some of the electronsof the much larger electron flow in the opposite direction. In the IGBTon state, this second local NPN bipolar transistor turns on and injectselectrons from the floating N+ type well region 13 vertically downwardas indicated by arrow 35. The extra electron injection afforded by thefloating well structures reduces V_(CE(SAT)). The precise nature ofcarrier flow may not be as simple as described above, but the overallelectron-injecting phenomenon and effect has been verified by simulationusing the ISE-TCAD device simulator available from Synopsis, Inc., 690East Middlefield Road, Mountain View, Calif. 94043.

In the IGBT off state, the voltage on the gate electrode 14 is such thatthere is no conductive channel through the P type body region 7 alongthe vertical sidewall edge of the trench 17. Electrons therefore cannotpass from the N+ type emitter region vertically down through any channelto the N− drift layer 6. Holes are therefore not injected upward acrossthe PN junction between layers 5 and 3 in bipolar IGBT action. Becausethere is no electron flow from the N+ type emitter region verticallydown through any channel, current flow through the floating N and P typewell regions that occurs in the IGBT on state to inject additionalelectrons into the N− drift layer does not occur in the IGBT off state.Accordingly, there is no current flow between the IGBT collectorterminal and the IGBT emitter terminal, and the IGBT device is off.

In the off state of the IGBT structure, there may be a high reversevoltage present across the device between the collector and the emitter.The floating P type well region 12 is made thicker where it is adjacentthe trench 17. The depth DP2THICK of the floating P type well region 12at this location adjacent the trench is substantially the same as thedepth DP1 of the P type body region 7 on the other side of trench 17.Due to the floating P type well region 12 and the P type body region 7being deep in these areas adjacent to the trench, the curvature of theelectric field under high reverse voltages in the IGBT off state isrelaxed. The less-sharp curvature of the electric field under the trenchand at the bottom corners of the trench 17 serves to increase thereverse voltage at which the IGBT suffers reverse breakdown.

FIGS. 4A through 4V are sets of diagrams that illustrate a method ofmanufacturing the IGBT die structure of FIG. 1. An N− type drift layer 6is disposed over the top major surface of an N+ type buffer layer 3. Inone example, the N+ type buffer layer 3 is a layer of semiconductorsubstrate of monocrystalline wafer material and the N− type drift layer6 is formed on the top major surface 4 by epitaxial deposition. Thisstructure, as shown in FIG. 4A, is the starting material for theprocess. Next, an oxide layer 100 is formed on the upper surface of thewafer as shown in FIG. 4B. Next, a ring mask 101 is formed. Theseparations between the features of the ring mask 101 determines thespacing between the thicker portions of the floating P type well regionto be formed, and also determines how shallow the thinner portions ofthe floating P type well region will be. An extra mask is not necessaryto form the waved bottom boundary of the floating P type well regionbecause the ring mask 101 of FIG. 4C is used in the IGBT manufacturingprocess to form the floating P type rings in the edge termination areaof the die. In accordance with one novel aspect, addition features ofthis ring mask are provided in the active area so as to define and toform the thinner portions of the floating P type well region as shown inFIG. 4C. After implantation using this ring mask 101, the ring mask isremoved as shown in FIG. 4D, and the P type dopants are diffuseddownward in an annealing step as illustrated in FIG. 4D to form P typeregion 102. Next, a thicker oxide layer 103 is deposited as shown inFIG. 4E. A trench mask 104 is formed, and the oxide 103 is etched asillustrated in FIG. 4F. The trench mask 104 is then removed, and thepatterned thick oxide 103 is used as a mask to etch trench 17 asillustrated in FIG. 4G. This forms the floating P type well region 12and the P type body region 7. The patterned oxide layer 103 is thenremoved as illustrated in FIG. 4H. Next, a thin oxide layer 105 isformed as illustrated in FIG. 4I. Polysilicon 106 is then depositedeverywhere, including in the trenches, as illustrated in FIG. 4J. Thepolysilicon layer 106 is then etched away, such that polysiliconelectrode 14 is left filling the trench, as shown in FIG. 4K. An N+ mask107 is formed, and N type dopants are implanted as shown in FIG. 4L. TheN+ mask 107 is then removed, and the N type dopants are diffused in ananneal step as illustrated in FIG. 4M. This forms the floating N+ typewell region 13 as well as the N+ type emitter region 11 as shown in FIG.4M. The upper surface of the die is then planarized, and a layer ofoxide 109 is deposited, as illustrated in FIG. 4N. A contact mask 110 isthen formed, and the contact mask is used in a contact etching step toform contacts down to the semiconductor surface as illustrated in FIG.4O. This patterns oxide layer 106 into oxide layer 21. The contact mask110 is then removed and P type dopants are implanted as shown in FIG.4P. The P type dopants are then diffused in an anneal step to formcontact region 10 as illustrated in FIG. 4Q. Metal is deposited, masked,and patterned to form the emitter metal electrode as illustrated in FIG.4R. Backside grinding is then performed to thin the N+ type substratelayer 3 as illustrated in FIG. 4S. P type dopants are implanted from thebackside into the thinned N+ type substrate layer 3 as illustrated inFIG. 4T. The P type dopants are diffused in a backside laser annealingstep as illustrated in FIG. 4U to form P++ type collector layer 5. N+type silicon of the prior N+ substrate becomes the N+ type buffer layerof the IGBT. Metal is then deposited on the backside of the wafer toform the collector terminal 23 as illustrated in FIG. 4V. The uppersurface of the wafer is then covered with a passivation layer 18 asillustrated in FIG. 4W. The resulting wafer is then singulated intoindividual IGBT dice. The individual IGBT dice are packaged and tested.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. The floating N+ type well region may have variousdifferent shapes and thicknesses in various embodiments, and does notneed to entirely separate the floating P type well region from the uppersemiconductor surface in all embodiments. The floating N+ type wellregion can have more than one thickness. The floating N+ type wellregion need not, in all embodiments, form any part of a sidewall of thetrench. In the same IGBT dummy cell, some of the localelectron-injecting NPN transistors can have thicker bases whereas othersof the local electron-injecting NPN transistors can have thinner bases.The floating P type well region and the floating N type well region aredesigned together so that the resulting thicknesses of the bases of thelocal electron-injecting transistors are proper for obtaining andmaintaining the desired charge balance between electrons and holesacross the lateral dimension of the IGBT device when the IGBT is in theon state. The N type dopant concentration immediately beneath the trenchin the N− type drift layer may be increased (for example, by dopantimplantation through the bottom of the trench) in order to increaseelectron flow laterally under the trench, if such increased electronflow is desired. Where the P type body regions of the device are formedto have areas of higher P type concentration, the same processing stepsused to form those areas can also be used to form areas of higher P typeconcentration in the floating P type well regions. For additionaldetail, teachings, structures and methods, see: U.S. patent applicationSer. No. 14/840,322, entitled “IGBT With Waved Floating P-Well ElectronInjection”, filed on Aug. 31, 2015, by Kyoung Wook Seok (the entiresubject matter of which is incorporated herein by reference).Accordingly, various modifications, adaptations, and combinations ofvarious features of the described embodiments can be practiced withoutdeparting from the scope of the invention as set forth in the claims.

What is claimed is:
 1. An Insulated Gate Bipolar Transistor (IGBT) diestructure comprising: a P type collector layer; an N− type drift layerdisposed over the P type collector layer; a P type body region thatextends into the N− type drift layer; an N+ type emitter region, whereinthe N+ type emitter region extends into the P type body region from asubstantially planar upper semiconductor surface; a floating P type wellregion that extends into the N− type drift layer and that is laterallyseparated from the P type body region, and wherein the floating P typewell region has at least one thinner portion disposed between two of aplurality of thicker portions; a floating N+ type well region thatextends into the floating P type well region from the substantiallyplanar upper semiconductor surface, wherein the floating P type wellregion is separated from the substantially planar upper semiconductorsurface by the floating N+ type well region, and wherein the floating N+type well region extends over the at least one thinner portion; a trenchgate electrode disposed in a trench, wherein the trench gate electrodeis disposed at least in part between the P type body region on one sideof the trench and the floating P type well region and the floating N+type well region on an opposite side of the trench; a first metalterminal, wherein the first metal terminal is coupled to the P type bodyregion and to the N+ type emitter region; a second metal terminal,wherein the second metal terminal is coupled to the trench gateelectrode; and a third metal terminal that is coupled to the P typecollector layer.
 2. The IGBT die structure of claim 1, furthercomprising: an N+ type buffer layer disposed between the P typecollector layer and the N− type drift layer.
 3. The IGBT die structureof claim 1, wherein the at least one thinner portion of the floating Ptype well region is less than half as thick as each of the thickerportions of the floating P type well region.
 4. The IGBT die structureof claim 1, wherein the trench extends from the substantially planarupper semiconductor surface to a depth DT, wherein the P type bodyregion extends to a depth DP1 measured from the substantially planarupper semiconductor surface, wherein the thicker portions of thefloating P type well region extend to a depth DP2THICK measured from thesubstantially planar upper semiconductor surface, wherein DT is greaterthan DP1, and wherein DT is greater than DP2THICK.
 5. The IGBT diestructure of claim 4, wherein the floating P type well region forms apart of a sidewall of the trench, wherein the floating P type wellregion at the sidewall at a location immediately adjacent the trenchextends to the depth DP2THICK.
 6. The IGBT die structure of claim 4,wherein the floating N+ type well region has a polygonal outer peripherywhen the trench IGBT die structure is considered from a top-downperspective, wherein the floating P type well region has the samepolygonal outer periphery when the trench IGBT die structure isconsidered from the top-down perspective, and wherein the trench extendsaround the outer periphery of the floating N+ type well region and thefloating P type well region such that it surrounds the floating N+ typewell region and the floating P type well region when the trench IGBT diestructure is considered from the top-down perspective.
 7. The IGBT diestructure of claim 6, wherein the floating P type well region at alllocations along its polygonal outer periphery extends to the depthDP2THICK.
 8. The IGBT die structure of claim 6, wherein the polygonalouter periphery of the floating P type well region has a shape takenfrom the group consisting of: an octagon, a square with rounded corners,a rectangle, a rectangle with rounded corners, a rectangular strip. 9.The IGBT die structure of claim 6, wherein the N+ type emitter regionforms a sidewall of the trench, and wherein the N+ type emitter regiondoes not entirely loop around the polygonal outer periphery of thefloating N+ type well region and the floating P type well region whenthe trench IGBT die structure is considered from the top-downperspective.
 10. The IGBT die structure of claim 6, wherein the N+ typeemitter region is a first N+ type emitter region, wherein the IGBT diestructure further comprises a second N+ type emitter region, a third N+type emitter region, and a fourth N+ type emitter region, wherein thefirst N+ type emitter region extends along a first planar sidewall ofthe trench, wherein the second N+ type emitter region extends along asecond planar sidewall of the trench, wherein the third N+ type emitterregion extends along a third planar sidewall of the trench, and whereinthe fourth N+ type emitter region extends along a fourth planar sidewallof the trench.
 11. The IGBT die structure of claim 1, wherein a firstthinner portion of the floating P type well region has a substantiallypolygonal shape when the IGBT die structure is considered from atop-down perspective.
 12. The IGBT die structure of claim 11, wherein asecond thinner portion of the floating P type well region has asubstantially polygonal shape when the IGBT die structure is consideredfrom the top-down perspective, wherein the second thinner portion loopsaround the first thinner portion such that the first and second thinnerportions are concentric polygons when considered from the top-downperspective of the IGBT die structure.
 13. An Insulated Gate BipolarTransistor (IGBT) die structure comprising: a P type collector layer; anN− type drift layer disposed over the P type collector layer; a P typebody region that extends into the N− type drift layer; an N+ typeemitter region, wherein the N+ type emitter region extends into the Ptype body region from a substantially planar upper semiconductorsurface; a floating P type well region that extends into the N− typedrift layer and that is laterally separated from the P type body region,wherein the floating P type well region has a thinner portion disposedbetween two of a plurality of thicker portions, wherein the thinnerportion of the floating P type well region is less than half as thick asthe thicker portions of the floating P type well region, wherein thethinner portion of the floating P type well region extends to a depthDP2THIN measured from the substantially planar upper semiconductorsurface, wherein the floating P type well region has a polygonal outerperiphery when the trench IGBT die structure is considered from atop-down perspective, and wherein the floating P type well region at alllocations along its polygonal outer periphery extends to a depth greaterthan DP2THIN measured from the substantially planar upper semiconductorsurface; a trench gate electrode disposed in a trench, wherein thetrench gate electrode is disposed at least in part between the P typebody region on one side of the trench and the floating P type wellregion on an opposite side of the trench; a first metal terminal,wherein the first metal terminal is coupled to the P type body regionand to the N+ type emitter region; a second metal terminal, wherein thesecond metal terminal is coupled to the trench gate electrode; and athird metal terminal that is coupled to the P type collector layer. 14.The IGBT die structure of claim 13, wherein the thinner portion of thefloating P type well has a polygonal shape when the trench IGBT diestructure is considered from the top-down perspective.
 15. The IGBT diestructure of claim 14, further comprising: means for conductingelectrons in an IGBT on state laterally in a plane that is parallel witha plane of the substantially planar upper semiconductor surface and forinjecting at least some of the electrons vertically down into the N−type drift region, wherein the at least some electrons that are injecteddown into the N− type drift region are injected at locations none ofwhich is disposed under the trench and none of which is disposed underthe P type body region.
 16. The IGBT die structure of claim 15, whereinthe means comprises a floating N+ type well region that extends adistance DN into the floating P type well region from the substantiallyplanar upper semiconductor surface, wherein the floating N+ type wellregion extends over the at least one thinner portion.
 17. The IGBT diestructure of claim 16, wherein the quantity DP2THIN minus DN is lessthan half the quantity DP2THICK minus DN.